This study was performed by Kathy Bevers and Per Kielland-Lund as
a requirement for Environmental Control Systems I, Fall 2003, at University
of Oregon. We decided to explore the thermal characteristics of cob
buildings, as cob is a building technique with obvious advantages
(low cost, simple construction, large thermal mass), while it also
has potential weaknesses (low insulative value). It is a technique
that is experiencing a revival, but there is a lack of research on
its characteristics and its appropriateness as a building technique
in various conditions.
We found our hypothesis in The
Hand-Sculpted House by Ianto Evans: Heat flows through cob
at the rate of an inch per hour. To test this hypothesis, we spent
two weekends at a 120 square foot cob cottage at Maitreya Ecovillage
in Eugene, Oregon. We installed temperature sensors at one inch increments
in the cob wall, fired a wood stove for 12 hours, and gathered data
at 10 minute intervals for a total of 54 hours. The data showed that
the peak temperatures at the various depths in the wall were staggered,
with deeper sensors recording peaks later than shallower sensors.
The time lag was equivalent to a heat flow rate of 2.7 inches per
hour. This is a significantly higher rate than our hypothesis states,
but may partly be explained by an indoor temperature during the heating
period that was significantly higher than typical indoor temperature
(90 to 100 degrees Fahrenheit, compared to typical indoor temperatures
of 65-70 degrees Fahrenheit).